1. Field of the Invention
The present invention relates to a hydraulic driving apparatus provided in a working machine, such as a crane, to drive a load, such as a suspended load, in the same direction as a self-weight falling direction, i.e., a direction along which the load falls by its self-weight.
2. Description of the Background Art
As an apparatus for driving a load in the same direction as a self-weight falling direction of the load, there is known, for example, a lowering drive apparatus for driving a winch which suspends a load by a wire rope, in a lowering direction. For this apparatus, it is important to prevent falling of a suspended load due to stalling of a winch motor caused by cavitation arising from a lowering in pressure on a meter-in side during a lowering drive mode.
As means to prevent such a reduction in pressure on the meter-in side, JP 2000-310201A discloses a technique of providing a so-called externally-pilot-operated counterbalance valve in a flow passage on a meter-out side. This externally-pilot-operated counterbalance valve is operable to narrow the flow passage on the meter-out side when the pressure on the meter-in side becomes equal to or less than a set pressure thereof to thereby prevent pressure on the meter-in side from its excessive lowering.
The externally-pilot-operated counterbalance, however, has a pressure measurement point and a pressure control point which are located on the meter-in side and on the meter-out side, respectively; in other words, it is subjected to control missing so-called co-location under the control theory in which positions of measurement and control points are different from each other, thus having a problem of being fundamentally unstable and likely to cause hunting.
As means to prevent the above hunting, there exists a technique of providing an orifice capable of giving large attenuation to a valve opening movement of the counterbalance valve, in a pilot fluid passage, however having a problem that the orifice prolongs a valve opening time of the counterbalance valve to thus deteriorate response of the counterbalance valve, and further generates a large flow resistance in the counterbalance valve until it is fully opened, to thereby cause an unnecessary boosted pressure.
As another technique for preventing the hunting, the JP 2000-310201A discloses a communication valve controlling fluid communication between the flow passage on the meter-in side and the flow passage on the meter-out side and a flow regulation valve controlling a meter-in flow rate so as to make a pressure difference between the two flow passages be smaller; however, this technique has difficulty in obtaining a stable lowering speed. Specifically, in a general lowering control circuit, there is generated a holding pressure corresponding to a weight of a suspended load, which makes a pressure difference between meter-out and meter-in sides be larger as the weight of the load becomes larger, this increase in the pressure difference involving an increase in an opening degree of the flow regulation valve on the meter-in side and thereby increasing the meter-in flow rate. In the above conventional apparatus, the lowering speed will be thus largely changed depending on the weight of the load.